Input protection circuitry is an essential part of modern integrated circuits, particularly in the area of field effect transistor based devices. Extremely small delicate device structures are very sensitive to high voltages. The threat of high voltage damage begins during fabrication of integrated circuits into products and continues throughout the service life of the integrated circuits.
High voltage damage from electrostatic discharge may occur during installation of integrated circuits into products. Sources of electrostatic charge include the human body, within which charge is generated, and installation tools having residual charge. Electrostatic charge may be transferred to integrated circuits through physical contact. Subsequent to installation, high voltage damage is usually the result of a power surge. These high voltages may destroy integrated circuits, thus requiring expensive and tedious repairs on fully manufactured devices which could have been avoided by providing a mechanism for dissipation of the high voltage on the integrated circuit. This problem is particularly acute in complimentary metal oxide semiconductor field effect transistor type integrated circuits. In high density CMOS devices, an extremely high voltage may be developed by electrostatic discharge which easily destroys the very thin gate oxides and very short channel devices of the integrated circuit.
One way to protect against these over-voltage conditions is to use a silicon controlled rectifier (SCR) structure compatible with complementary metal oxide semiconductor (CMOS). This structure is shown in application Ser. No. 07/213,499, filed Jun. 27, 1988 now U.S. Pat. No. 5,012,317 and assigned to the assignee of this application. This structure provides excellent protection but the SCR protection device has a very high threshold voltage. This threshold voltage is commonly on the order of 70-80 volts. It has been found in many present day circuits that a 70-80 volt threshold is inadequate to protect some devices on the integrated circuit. Another input protection structure which greatly reduces the threshold voltage of the input protection is shown in U.S. Pat. No. 4,939,616, assigned to the assignee of this application, which shows an input protection device with a low trigger threshold. This structure is a silicon controlled rectifier (SCR) type of device wherein the triggering mechanism is avalanche conduction at the interface between the N-well surrounding a portion of the protection device and the P-type substrate.
Therefore, while input protection structures exist, it is highly desirable to provide an input protection structure which greatly increases the over-voltage protection that is available in present input protection structures.